The present invention is directed, in general, to wireless communications devices and, more specifically, to an RF receiver having a lower signal-to-noise ratio.
Wireless communications systems, including cellular phones, paging devices, personal communication services (PCS) systems, and wireless data networks, have become ubiquitous in society. Wireless service providers continually try to create new markets for wireless devices and to expand existing markets by making wireless devices and services cheaper and more reliable. The price of end-user wireless devices, such as cell phones, pagers, PCS systems, and wireless modems, has been driven down to the point where these devices are affordable to nearly everyone and the price of a wireless device is only a small part of the end-user""s total cost. To continue to attract new customers, wireless service providers concentrate on reducing infrastructure costs and operating costs, and on increasing handset battery lifetime, while improving quality of service in order to make wireless services cheaper and better.
To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base station (BS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
CDMA technology is used in wireless computer networks, paging (or wireless messaging) systems, and cellular telephony. In a CDMA system, mobile stations (e.g., pagers, cell phones, laptop PCs with wireless modems) and base stations transmit and receive data in assigned channels that correspond to specific unique codes. For example, a mobile station may receive forward channel data signals from a base station that are convolutionally coded, formatted, interleaved, spread with a Walsh code and a long pseudo-noise (PN) sequence. In another example, a base station may receive reverse channel data signals from the mobile station that are convolutionally encoded, block interleaved, modulated by a 64-ary orthogonal modulation, and spread prior to transmission by the mobile station. The data symbols following interleaving may be separated into an in-phase (I) data stream and a quadrature (Q) data stream for QPSK modulation of an RF carrier. One such implementation is found in the TIA IS-95 CDMA standard. Another implementation is the TIA S-2000 standard. The order of the Walsh code spreading or 64-ary modulation and PN spreading does not affect the performance of the present invention disclosed below in the DETAILED DESCRIPTION OF THE INVENTION.
A data bit equal to Logic 1 in the convolutionally encoded symbols is transmitted as one 64-chip Walsh code and a data bit equal to Logic 0 is transmitted as the inverse of the 64-bit Walsh code, obtained as an exclusive-OR (XOR) between the data bit and the Walsh code.
In order to increase the reliability of CDMA receivers, base stations and mobile stations frequently transmit M copies of the same signal, staggered in time, to the other device. The receiving device typically uses multiple receive paths, such as in a rake receiver, to capture each of the copies. The captured copies are summed to produce a composite signal in order to improve the signal to noise ratio. This allows the composite signal to be more easily de-spread and recognized by a signal correlator or matched filter. However, this approach requires a large number of components and a large circuit area. Additionally, the repeated transmission of M copies of the same signal is wasteful of scarce bandwidth.
There is therefore a need in the art for improved CDMA systems that have an improved signal-to-noise ratio in the receiver. In particular, there is a need for CDMA systems that do not require the transmission of multiple copies of a signal from a transmitter to a receiver. More particularly, there is a need for an improved CDMA receiver capable of improving the signal-to-noise ratio of a single copy of a received signal.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in a CDMA receiver, a noise reduction circuit for improving a signal-to-noise ratio of a received signal comprising a series of chip sequences. In an advantageous embodiment, the noise reduction circuit comprises: 1) a sampling circuit for generating an original plurality of samples of the received signal; and 2) a controller capable of determining a first plurality of time slots, each of the first plurality of time slots comprising a plurality of chip samples corresponding to Logic 1, and a second plurality of time slots, each of the second plurality of time slots comprising a plurality of chip samples corresponding to Logic 0, wherein the controller is capable of generating a reconstructed plurality of samples by at least one of: a) modifying an order of a first Logic 1 chip sample and a second Logic 1 chip sample; and b) modifying an order of a first Logic 0 chip sample and a second Logic 0 chip sample.
CDMA provides the foundation for the present invention since the coding and spreading sequence (i.e., chip sequence per bit) are known by both the transmitter and receiver.
In one embodiment of the present invention, the controller adds the reconstructed plurality of samples and the original plurality of samples to generate a composite signal having a reduced signal-to-noise ratio.
The controller uses a correlator or matched filter to de-spread the received signal and the composite signal. If the matched filter or correlator output does not indicate a signal match with the desired code, the controller generates a new pseudo-signal, sums it with the previously generated pseudo-signal sum and repeats the de-spreading function. The process repeats until the correlator output indicates a signal match with a signal-to-noise ration (Eb/No) greater that a prescribed threshold or until the execution of a set number of cycles. This process performs coherent combination of the samples that represent the digital data signal states since the signal states are the amplitude +a (for a chip state of Logic 1) and the amplitude xe2x88x92a (for a chip state of Logic 0). The noise components combine with random phase. Therefore, the larger the number of summed pseudo-signals, the smaller the noise contribution.
According to another embodiment of the present invention, the CDMA receiver is a receiver in a base station of a wireless network.
According to still another embodiment of the present invention, the CDMA receiver is a receiver in a mobile station capable of communicating with a wireless network.
According to yet another embodiment of the present invention, the first Logic 1 chip sample and the second Logic 1 chip sample are contained within a single chip.
According to a further embodiment of the present invention, the first Logic 0 chip sample and the second Logic 0 chip sample are contained within a single chip.
According to a still further embodiment of the present invention, the first Logic 1 chip sample and the second Logic 1 chip sample are contained within different chips and the first Logic 0 chip sample and the second Logic 0 chip sample are contained within different chips.
According to a yet further embodiment of the present invention, the controller one of modifies the order of the first and second Logic 1 chip samples and modifies the order of the first and second Logic 0 chip samples according to one of a random process algorithm and a predetermined algorithm.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.